Method and apparatus for measuring dynamic frictional properties of elastomers



v5--solenoid is located between-the tine-s to vibrateV the fork,= i

DBsCRrPTioNf-FPREFERRBEMBODIMENTS'e An undamped tuning fork does notfexhibit anysigii- `v` cant vibrationfbv or below'ts`-vibra`tion frequency "fammi I l Thus, a Agraphic representation of frequency against iwamplitu'der ofvibratiomis inV tlieformoa spikeV plot as shown in FIG. 1 Where the only appreciablearnplitude of `vibrationoccurs' at f (f0,k)."1liis undamped natural frequency 1in.' cycles jef-'second' Y("c;ii.s.)" cjrresptSndiig 'the fundamental AmodeA ofmvwibration will approigimately follow the equation I 'vi empio in vibrato imembersand, moreparticulady, vto f Y f' N n g the uef such crlevices and methods to measure 'them wh K l the .stfffless and m lshfhe Thassf" u irc pertiesof testiecesQas a functionV otltheir effectlon 1' f1' em* amfpmls pres-ent'suc as' at' contnbuted by y vibratio charactegtics. 3a t e-elastomer rubbing against the fark stylus, tbe resonant vfrequency will approximately follow the equation 'Descriptiorijof the prior art In the'yevluation of rubbers and elastomeric materials it is'iofteiiwlig'bly de's'irbl kn'ow' what the dynamic frictiofial characteristics of -tlief-materials V4are' without tbe `nelfessityof makinga cmpletefabrication of an article,

. like a tire, with subsequent actual performance tests.

'Moreoven 'it is even mo'redes'irable to determine vwhat Under these conditions-the memberis yarriplitudeA oi" the 'vibrating n j* In U.S. Patents A2,733,596-and 3,030,803 to Painter,

" devices are described wlie're'ira vibrator-y4 device isused to" i..evaluate.elastomers-by measuring tlie elastic-anddamping modulus. iBradield,v U.S. Patent 2,8(B',129,-describes a resonantv sensing'devce which measures surface Propr erties ofwm'aterials, such as surface compliance andantechanicalim'pedaiige as'afunfctiofrr ofthe 'shift'in' resonantV l -None Qtthepricrartteashinss prolidea measure dynamic frictiorrofelastomeric.materials has been found that a vibratory sensing device can. be used to determine the dynamic frictionof a rubber, svhich device.comprises (l) a vibratory member liaving a `as described above. Awplot of A over the test range of frequencies will produce"theereson'ancefcurvesliown'in' "e" 5 FIG. L .lj the danping force is appercablc,.the plot will be bell-shaped; vIf onemarks the frequencies f1 and fg,

which-"ges @fill-1&7. times the meiflium amplitude...

,fthe erence between f1 and. `is Af andithinsiis i knownV as the ohef'lialf power ban'dividthflt is'v called 'this D5 because theamplitudes at 0.707 timestlie maximum ampingeraepfeieiieneiaifsaefm 'j 1j 'einem aiguise f0fe s r.s5tsd ly.1 tflfat tuninsiorlclirte-llbsins presse against te rubbervte'stpiece llrassliown/n FIGS.- 2 and tand the stylusflt)A vibrates'laterallyA as shown-fthe bandwidth Af j i will be a measure of the dynamic friction caused bytheA l l 'rubber test'piecelt can befshown' that theabsolute'value' ofthe clamping term cori'espondiifigto dynamic friction is Y' where a is a .proportionalityffactor.-Since bandwidth will change somewhat with increases in temperature for the n same test piece, comparative experiiiieiits slio'ul'dbe`A run A' at the same temperature (e.g., room temperature).

'The dynaniicfriction measured Willfirxclude both an adhesive component and a hyster'etic component. To

` current 'frequency,` 'one can drive to the fork.

elimina-te theadhesive component and measure .it-he vhys 'i' 4teretic.only, one can lubricate the stylus point with bentonite clay or preferably by placing a thin low-friction sheet, such as Teflon fluoro-polyrner, over the rubber test pieceat the pointv where the stylus touches it.

Although this device carin-be adaptedto jshow`the change in maximum amplitude of vibration or the change in'frequency at maximum amplitude, in comparisonr to the fork itself or some other test piece; the measurement of"n dynamic friction is not a single or simple function of these factors and they need not be measuredby thisde-4 yice; It might, however, bedesirable to adapt thexlevice to accomplish this so that determinations such as dynamic'modulus, etc., can be made in'addition to' dynamic v frietione# V1 I Referring now to the drawingsg particularly FIGS. '3, -t and 5, it can befseenvthatY a pair of1j ste e lstyl`r 10 are attached to the tines of a tuning Yfork 12'. A fork havinga frequency of `vibration vat 50 wcycles per second (c .p.s.)

is'siitable. Mounted onthe side of 'one of` the tines 4is l a piezoelectric crystal 13Alead-tit'riatrystiil attached by means of a conductive epoxy cement can be used. A

depending o nwth'e relative values of the 's'li'fin'essor damp'- ingLThe gear train 20 varies theV frequency of the'cuir'rentf supplied by generator 22 to the solenoid 17. This causes the fork 12 to be driven at frequencies within this range.

5` The amplitude of vibration will vary as shown bythe 'bell-shaped plot in FIG; 1. This amplitude will be sensed urementfof thepictureY on thescopeQlt'sVn possible to photograph the scope curve -and make the measure-- met'snfrom the photo. A more sophisticated form of the device .could include means for sensingithe two points on thesweep Aat which the crystal was signalling an ampli-l tude'of 0.707 -times the maximum, vand start a count to Y readutbahdwidth directly.

support platf9r m 14 is rattached to` the fork 1 2 and eirY tends out under the tines to hold the rubber testpiece`1`1 shown. It is desirable that the test piece 11 be held The apparatus can be calibrated to give direct read wvings inl terms of absolute dynamic friction. It is most often nsedf'loweve'r, as a comparative Ydevice tocompare the dynamic friction as a function of bandwidth of a test under about 10% compression, This israccomplished by piece withthat of a test piece of a rubber composition cover piece 15, which has a hole for the Styli 10, and is supported by suitably dimensioned blocks 16. The assembly is held into place by conventional clampingmeans (not shown). The displacement or penetration of the styli 1t) into the test piece 11 should be kept constant. A

typical test piece 11 is a slab 2 in. by l in. by 0.08 in.

The styli penetrate and defo-rnrtbutfnotpunc'tureth test piece about 0.018 in. in the 0.08 in. thickness. Each contacting the rubber. The measured value of A'dynamic friction is a function of theY area of contact between the tips of the Styli and the rubber. For this reason it will increase or decrease as the penetration'increases or de- A solenoid 17 is mounted between the tines of fork'v12. One end is fixed near the bas of the fork, and' the other end is' spaced from the ltines as shown in FIG. 3; When an alternating current is passed through rhe'solenoid '17, the varying ux between the tines will act as a vibration driving force at the particular frequency of current alternation. By employing means to adjust or varythe Y 'apply a `variable 'frequency The p piezoelectric crystal'13 will4 generate -a charge whose voltage, ranging from about 0.1'to 10 volts, will be proportional to the amplitude of vibration of the fork. *the output from this 'crystal- 1'3 is-connected toY Y Athe vertical amplification .terminal o f an oscilloscope 1 8,

The horizontal amplification terminal of theOScillowhose actual frictional performance is known (e.g., styrene-butadiene or natural rubber compositions).

- A's-'stated-abover when the styli are presscdfAagainstY-n.

the test piece in an unlubricated condition, the bandwidth Af is a measure of adhesive friction plus hysteretic friction. To measure the degree of each, the bandwidth is measured-erraten' piece first in aedryfand next ira Ylubricatedcondition; the hysteretic component is thus f stylus is'jabout00`5V in. in diameter with a rounded tip -35 vrneasured -directly in the latter reading, and the adhesive j 0 ample, when it is found that a certain factor (e.g.,

amount of Lpartilesiz`e'- o'f carbvo'black) has a proportional effect on the dynamic friction of thefrubber composition, then 'measurement of dynamic friction can be a measure'of this other factor. If the-rubber composition is not homogeneous,the rubber contacting one stylus may differ in composition from the rubber contacting the other stylus. Tlius,A bell-shapedresponse curves created 'by each"styluswill`differ inshape and/or a resonant frescope is connected vto-a variable potentiometer 19.'The

needle of the potentiometer 19 is connected to a gear train 20, driven by a constant speed clock motor 21. The

quency. Howeversince there Vis'but one curve determined .and depicted on the oscilloscope, it will be the product 0f 'thetwo a'irtd'appear as a single bell-shaped curve withV a hump one side ofthe peak. The device is thus capable of determining the non-homogeneity of the-compounded rubber Ystock whenev/er such a hump appears.

A device which can be'u'sedV 't''deterrnine both the i adhsiveconponent and hysteretic component of dynamicY friction is very valuable."liorexarrzple.V it enables" gear train is arranged so that there will be. an alternating sweep from A to B (i.e., a sweep of about 0 to lOyolts) everytwominutes.

The gear train 20 simultaneously drives the selector on a variable frequency (50 to 70 c.p.s.)"'osci1lator` generator ,22`(e.rg., a Hewlett-Packard model 202A).

The output fromthe generator 22 is passed to an aniplier 23 (e.g., a Bogen PA) which is in turn connected cross the terminals of solenoid 17.

The apparatus operates as follows: the tuning fork 12 has a damped resonant frequency of maximum amplitude Y within the approximate range from 5l to 70 cps. (e.g.,

"0 60 c.p.s.) for the particular fork used. When damped by t' oneto find from a very simple test whether a tread stock would :egthibit*gpodjractionv on wet pavement, which is dependent primarily on the hysteretic component alone. .i Wh'atis claimed is: i

1. Avibratory sensing device which comprises:

" (a) a vibratory member` having a stylus attached s thereto,

'f(b)'r nea'ns'for holdingsaid stylnsin deforming con tact with the surface of a test piece,

l(c) means for inducing vibratory motion of said member over a range of frequencies including theV resonant A frequency, said motion imparting a lateral vibratory movement of the Stylus with respect to said test piece i surface,A

(d) means for sensing the amplitude of vibration of said member over said frequency range, and

' (e) means for sensing the difference in frequencies at which the vibration amplitude is 0.707 times the .-.-..,....(b)V means forwholding saidstylus in deforming con- A-rf-f'---(iwmeanstofcontinneuslyfvary the--frequency-of the 25 Y. "alternating current supplied by' said generator,'

(g) means to continuously sense the amplitude of vibration of said fork at the frequency causing said vibration and thereby sense the difference in fre- V a quengies at wbichetbe ,vibration @aptitude is 0.70] Y 30 JOHN i?.agAUHAMP,Assistantnrtaminer member is a turning maximum amplitude of vibration within said range.

Y2. -A--deviccas defined in-claiml'wherein saidtvibratory fork'hauling` said stylus attached to a A 3. A device as defined in claim 2` wherein said means for inducing motion lisa' solenoid locatedbetween the tines of saidffo'rk'.

v V4. Aedevice forYmeasuring-theldynamic-tfrction ofV a rubber test piece which comprises;

" V(c.) "alternating current solenoid located between the tines of said fork to induce vibratory motion of Saidtines overe-'af'rrange of` alternating current fre'V -ctie component of friction ismeasuredapart from-,the quencies including` ntite resonant frequency; Enid" Y lmotion imparting a lateral vbratorymovement of 'rr-2' .j vthestyluswith vrespect to-thetestpiece surface, (d) a piezoelectric crystal attachedto a tine of said 20 fork to produce an output voltage-'propo tine thereof, s r

tact withfthe surface of said test piece,

"'1jf^`" thelamplitudefof vibratin'o said'iork'" (e) an alternating current generator meansv to supply current to said solenoid over said frequency range,

ng -a stylus..- attached to a times the maximum amplitude of vibration within Vsaidrangen 1 1;:-

-af SpA method ofV determing the dynamic friction Yof a rubber-test piece which comprises pressing' a stylus into l deforming `contact with Vthe surface of s'aid test piece,v vibrating an elongated mechanical member connected to said `stylus over .a range of fre,quex 1 cies,V including the resonant frequency, in adirection that causesvlateral vibration vof the stylus with respect t'o `said test piece t surfacer'sensing. the amplitudeof vibration of sidmember-over said frequency range,- and measuring the ditfer-= .ence in frequencies .wat nwhich .the vibration-amplitude is i., 0.707 times the-maximum amplitude -of vibration within t 44said range." f l 61. The method defined in claim- 5 wherein the hysterntthcsin'e'component by lubrcating's'aid test piece. at its! 1vv -pointof contact'with said stylus. t

Reterencesncited y Kleesattel 73-67.1

RICHARD C. QUEissBR, Primary Examiner 

